Method of burning fuels by means of a burner

ABSTRACT

A method of burning a fuel by means of a burner which comprises premixing the fuel with air in an amount larger than the quantity theoretically required for the combustion of the fuel, allowing the resulting lean premixture to be issued from some of plural nozzles of the burner, while at the same time, premixing the fuel with air in an amount less than the theoretically required quantity, allowing the resulting rich premixture to be issued from the remaining nozzles, and burning the two gaseous premixtures together.

United States Patent [191 Takahashi et al.

[451 Sept. 23, 1975 METHOD OF BURNING FUELS BY MEANS OF A BURNER [75]Inventors: Yasuro Takahashi; Yukihisa Fujima; Hisao Yamamoto, all ofNagasaki. Japan [73] Assignee: Mitsubishi Jukogyo Kabushiki Kaisha,Tokyo, Japan [22] Filed: Feb. 14, 1974 [21] Appl. No.: 442,679

[30] Foreign Application Priority Data Feb. 24, 1973 Japan 48-22339 [52]US. CL, 431/2; 431/4; 431/115; 431/285 [51] Int. Cl. F23C 11/00 [58]Field of Search 431/2, 9, 3, 278. 8, 285, 431/10, 12, 4,11,115;60/D1G.l1

[56] References Cited UNITED STATES PATENTS Paul ct al. 431/8 X 4/1973Schwedersky 431/8 7/1973 Stengel 431/4 Primary Examiner-Edward G. FavorsAttorney, Agent, or Firm-Toren. McGeady and Stanger [57] ABSTRACT Amethod of burning a fuel by means of a burner which comprises premixingthe fuel with air in an amount larger than the quantity theoreticallyrequired for the combustion of the fuel, allowing the resulting leanpremixture to be issued from some of plural nozzles of the burner, whileat the same time, premixing the fuel with air in an amount less than thetheoretically required quantity, allowing the resulting rich premixtureto be issued from the remaining nozzles. and burning the two gaseouspremixtures together.

9 Claims, 17 Drawing Figures US Patent Sept. 23,1975 Sheet 1 of 63,907,488

NOX FIG. 1 ippm) EXCESS AIR RATIO INSIDE BURNER NOX (p SOC EXCESS AIRRATIO INSIDE BURNER US Patent Sept. 23,1975 Sheet 2 of6 3,907,488

NOX (pp 1000- FIG.3

EXCESS AIR RATIO INSIDE BURNER FIGA FIG.5

GODDESS? UUEJDEIUEJ DUEIDEJEJDU DUEL JESSE] DUDDEJUEIE] EJUDEIEJDEIEIUEIUUUUUE! UUEJDEIUUU UCJUEJUUEIE] DUUUEJCIEJEI EIEIEIEIDEJEIUUUUEIUEIEIU US Patent Sept. 23,1975 Sheet 3 of6 3,907,488

FIG.9

US Patent Sept. 23,1975 Sheet4 of6 3,907,488

FIGM

US Patent Sept Sheet 5 of 6 mimosa,

US Patent Sept. 23,1975 Sheet 6 of 6 3,907,488

FIG.16

METHOD OF BURNING FUELS BY lVIEANS OF A BURNER This invention relates toa method of burning fuels by means of a burner, which can reduce thenitrogen oxide concentrations in the combustion gases.

More particularly, the invention concerns a method of burning gaseousfuels and light and heavy liquid fuels which comprises premixing,upstream of the burner, a fuel gas or vapor with air, with or withoutthe addition of an inert gas or liquid in two different amounts, overand below the amount theoretically required for the combustion of thefuel, and burning the two premixtures to produce a so-called premixflame, thereby effecting combustion while remarkably reducing theconcentrations of nitrogen oxides in the emissions and achieving a goodthermal efficiency.

In a combustion flame the oxides of nitrogen (hereinafter called NOx)are produced by the combination of oxygen and nitrogen molecules in thecombustion air. The velocity of NOx production depends in large measureupon the flame temperature, or the higher the flame temperature, thefaster NOx develop. it is therefore effective to decrease the flametemperature in order to reduce the NOX production as a result ofcombustion. This is well known in the art.

Conventional burners are generally of so-called dif fused flame typewhereby fuel gas and air for combustion are injected in a furnace by wayof separate passages.

in order to reduce the NOx contents of the combustion gases from suchburners, the following methods have been adopted:

1. Two-step combustion, so called because part of air supply forcombustion is issued from the burner to decrease the excess air rate inthe burner and the remaining air is released from a port independent ofthe burner.

2. Inert-gas mixing, whereby an inert gas (e.g., com bustion gas) isadded to the air for combustion.

3. Ununiform excess-air combustion, for which a plurality of burners areused with air supply at a uniform flow rate but with ununiform fuelsupply settings.

These and other methods hitherto proposed and practiced were examined byus for their NOx-reducing effects. Typical results are presented inFIGS. 1 to 3.

FIG. 1 shows the relation between the excess air rate inside a burner(on the abscissa) and the NOx concentration in the combustion gas (onthe ordinate) produced by the combustion of a gaseous fuel.

FIGS. 2 and 3 show the results of experiments, respectively, with alight liquid fuel (kerosene) and a heavy liquid fuel (heavy oil).

In connection with these figures, the term excess air rate inside aburner is used to mean the ratio of the amount of air that passes thethroat of the burner (in the two-step combustion, such an amount minusthe quantity of air blown in the latter period of combustion) to theamount of air theoretically required for the combustion.

The broken-line curves in FIG. 1 represent the results of experimentsconducted by using ordinary burners, with the ratio (hereinafter calledGM) of the amount of an inert gas (combustion gas) introduced into airto the amount of the air for combustion increased from GM to GM;,. Itwill be seen from the graph that the NOx concentration of the combustiongas continues to increase gradually until the excess air rate inside theburner reaches about 1.2 but remains substantially constant beyond thevalue of the excess air rate.

It can be seen from the experimental results given in FlG. 1 that thetwostep Combustion method limits the air supply to the burner andthereby reduces the excess air rate in the burner and to some extentdecreases the NOx concentration in combustion gas.

Also it is obvious that the addition of an insert gas, say combustiongas, is fairly effective in that the NOx concentration in the exhaustgas drops with an increase in the percentage of the inert gas (i.e.,with the decrease of the excess air rate inside the burner).

In the uniform excess-air combustion method, a plurality of burners areso set as to have irregular excess air rates and combustion is caused insome burner with a large excess of air (e.g., at din FIG. 1) and in theother with a small excess of air (e.g., at e in the same figure). Themethod is not appreciably useful because the effects of large and smallexcesses of air counteract and, as a whole, the resulting NOxconcentration is such as arising from the combustion with a mean excessof air, e.g., atfin the figure.

The present invention has for its object the provision of a method ofburning fuels and a burner therefor which can eliminate the drawbacks ofthe afore described methods.

To achieve the end the invention resides, firstly, in a method ofburning fuels, and a burner therefor, which comprises effecting thecombustion by discharging from some of plural nozzles of a burner a leanpremixture consisting of air in an amount more than that theoreticallyrequired for the combustion and afuel (i.e., a gaseous, liquid, orevaporated or atomized liquid fuel) and by issuing from the remainder ofthe nozzles a rich premixture consisting of air in an amount less thanthe theoretically required quantity and the same fuel.

Secondly, the invention pertains to a method of burning fuels as definedabove, wherein the gaseous fuel to be used is a gasified fuel obtainedby thermally decomposing a heavy liquid fuel, mixing the decompositionproduct with an inert gas or liquid, and then cooling the mixture. i

The method of the present invention will be better understood from thefollowing detailed description.

1. When a gaseous fuel is used, the method of the invention is reducedto practice in the following way: The gaseous fuel and air in an amountmore than that theoretically required for combustion are premixedupstream of a burner, and the premixed gas is jetted out from some ofplural nozzles of the burner, while, at the same time, the gaseous fueland air in an amount less than the theoretically required quantity arepremixed, and the premixture is issued from the remainder of the burnernozzles to effect combustion. it is also possible to add an inert gas orinert liquid to either or both of the gaseous premixtures before theyare delivered from the burner noules for combustion.

2. When a light liquid fuel is employed, the method is practiced asfollows: At least a part of the light liquid fuel is evaporated to acombustible gas or vapor, premixed with air in an amount more than thetheoretically required quantity, with or without the addition of aninert gas or liquid, and the resulting gaseous premixture is issued fromsome of plural nozzles of a burner for combustion. At the same time, atleast a part of the light liquid fuel is evaporated to a combustible gasor vapor, premixed with air in an amount less than the theoreticallyrequired quantity, with or without the addition of an inert gas orliquid, and the resulting gaseous premixture is burnt as it isdischarged from the remainder of the burner nozzles.

3. With a heavy liquid fuel, the method is further modified as follows:The fuel is thermally decomposed to a combustible gas or vapor, which isthen diluted and cooled by the addition of an inert gas or liquid (e.g.,water vapor), and a part of the resulting gas is premixed with air in anamount more than the theoretically required quantity, and then thepremixture is issued for combustion out of some of plural nozzles of aburner. Simultaneously, the remainder of the diluted and cooledcombustible gas or vapor is premixed with air in an amount less than thetheoretically required quantity, and then the premixture is spouted fromthe remainder of burner nozzles for combustion.

For the purpose of the invention, the amount of air more than thequantity theoretically required for combustion is, in terms of theexcess air rate (the amount of air actually'cbnsumed for combustiondivided by the amount theoretically required for completecombustion'),pr e'ferably between 1.3 and 1.6, and the amount of airless than the theoretically required quantity is an excess air ratepreferably between 0.5 and 0.8. Although, the amount of air issued fromall of the nozzles (i .e., the sum of air introduced to support thecombustion) is one to 1.2 times the amount of air theoretically requiredfor the complete combustion of the fuel discharged from all of thenozzles.

' The advantageous effects of the present invention will be described indetail hereunder.

1. The results of combustion experiments conducted with a gaseous fuel,using a burner adapted for practicing the method of the invention arerepresented by continuous-line curves (GM GM in FIG. 1.

As can be seen from the graph, the use of the burner in conformity withthe invention produces curves entirely different from those (representedby broken lines) conventionally obtained with an ordinary burner. Thecurvcs according to the invention are mountain. shaped with very sharpgradients, the peaks corresponding to excess air rates of approximately1.0 to 1.1.

If no inert gas is admitted and the excess air rate inside the burner isclose to 1.0, the NOx concentration in the combustion gas will be ratherhigh as compared with the values attained with conventional methods.However, as the excess air rate inside the burner increases, and also asthe inert gas addition to air assumes a larger percentage, the NOxconcentration will markedly decrease and, from a certain boundary pointon ward, drop below the values with the prior art methods; eventuallythe NOx concentration will be reduced to a minimum that is neverattained conventionally.

This is explained by the fact that, whereas the ordinary methods aredirected to socalled diffused flames, the method of the inventionproduces a so-called premix flame, and that the dissimilarity in flamestructure brings about different mechanisms for NOx formation.

As noted above, in the present invention, gaseous mixtures with variedfuel concentrations are supplied to different nozzles of a multi-nozzleburner, thus at taining ununiform excess air rates in the pluralnozzles. The beneficial effects thereby achieved will now be described.

Referring to FIG. 1, the point a is the actuating point of a nozzle setfor an excess air rate of more than 1.0, and the point b is thatactuating point of a nozzle set for an excess air rate of less than 1.0.

The both actuating points a and b give lower NOx concentrations than bythe corresponding points in the conventional methods. Combining thepoints a and b brings an overall excess air rate inside the burner as atthe point c. Thus, compared with an ordinary arrangement in which allburners are set to the point a, combustion with less excess air is madepossible. It follows that a heat exchanger equipped with the burnerincorporating the method of the invention can achieve a very highthermal efficiency.

The nozzle actuated at the point b would produce a flame withinsufficient air supply, or in a state of incomplete combustion. Thecondition can be improved, for example, by sandwiching a nozzleactuatable with an excess air rate at the point b with nozzlesactuatable at the point a, or vice versa. In this manner the combustionincomplete at the point b will be made complete by the excess air at thepoint a.

As an alternative, the air inside the burner may be mixed with an inertgas (e.g., the combustion gas) to shift the actuating point from a to aor a" and, likewise, shift the actuating point from b to b" or b".Accordingly, the average excess air rate throughout the burner will beshifted to the point 0' or c". This will make possible combustion with asmaller excess of air and with a higher thermal efficiency than when thecombustion is conventionally carried out with all burners set to thepoint a or a". Moreover, the NOx concentration in the combustion gaswill be practically reduced to naught.

2. The results of combustion experiments of a light liquid fuel(kerosene) with a burner adapted for practicing the method of theinvention are represented by continuous lines in FIG. 2. An alternatedash and dotline represents the result of an experiment with keroseneand an alternate dash and two dots line represents with result withheavy oil, both conducted in a conventional way. In the graph thecontinuous line A summarizes the result of an experiment in whichkerosene was completely evaporated, premixed with the total amount ofair supplied for the combustion purpose, and the premixture was burnt bya gase burner of the premix type.

It will be manifest from the graph that, when a light oil is evaporatedand premixed for combustion in some way as will be described later, theNOX production will be reduced to such a low level that is neverattained by the ordinary spray combustion method.

In FIG. 2 the continuous lines 8 and C indicate the effects of an inertgas (combustion gas) introduced into air upon combustion of kerosene inaccordance with the present invention. As compared with the air free ofcombustion gas (A), the air containing a small quantity (B) or a largequantity (C) of such inert gas is helpful in reducing the NOxconcentration in the combustion gas discharged; the greater the additionof such inert gas to air, the lower the NOX concentration will be.

In the combustion method of the invention, it is also possible todecrease the NOx concentration to extremely low levels as represented bystraight lines B, B and B" by effecting combustion with the addition ofthe same proportions of combustion gas as in A, A,

and A, respectively, but with very small excess air rates, or in thestate of insufficient air supply.

For this reason,.if a burner having a plurality of nozzles is employedand some of the nozzles are actuated at points a, a, and a" and theremainder at points I], b, and b, then the mean excess air rates at c,c, and c will permit small excess-air combustion with a high thermalefficiency while maintaining the NOx production at a low level.

While the instance in which combustion gas is mixed in air forcombustion has been described above, substantially the same effect isachieved by adding some other inert gas or liquid by spraying to air.

3. The relationship between the total quantity of NOx in combustion gasand the excess air rate in combustion of a heavy liquid fuel (e.g.,heavy oil) according to various burning methods is graphically shown inFIG. 3. The amount of an inert gas added was the same throughout theexperiments.

In the graph the curve of an alternate dash and two dots line representsthe result of a combustion experiment performed by use of a customaryspray gun.

l The curve of an alternate dash and dot line shows the result of anexperiment about so-called diffused-flame combustion effected bygasifying heavy oil and issuing the gaseous fuel and air separately fromnozzles of a common burner.

It will be clear from the graph that the combustion with gasificationinvolves conversion of less nitrogen content of the fuel into NOx andhence produces less NOx concentration in the exhaust gas than by thecombustion with spraying.

The continuous line gives the result of combustion with a burneraccording to Example 1 of the invention to be discussed later. It can beseen that NOx are formed in a manner quite different from the two casesreferred to above. In addition, the absolute level of NOx concentrationis by far the lowest, the peak being at the excess air rate ofapproximately 1.0.

Thus, if the excess air rate in some of adjoining gas passages, forexample in the gas passages l6"a in FIG. 17 to be considered later, islarge, for example at the point a in FIG. 3, while the rate in the othergas passages l6"b is small, for example at the point 19, then theoverall excess air rate inside the burner and the NOx concentration inthe combustion gas can be at the point c, for example.

In other words, the excess air rate is smaller and the thermalefficiency is higher than when all the gas passages are set to the pointa. Although incomplete combustion takes place in the gas passages 16"bdue to shortages of air, the combustion is made completely by the excessair in the adjacent gas passages 16"u.

The method and apparatus according to this invention are applicable toburners of boilers and other industrial furnaces as well as of gasturbines.

Other objects, advantages and features of the invention will becomeapparent from the following description taken in connection with theaccompanying drawings, wherein:

FIGS. 1 to 3 are graphs showing the relations between excess air ratesin burners and NOx concentrations in the combustion gases when gaseous,light liquid. and heavy liquid fuels are burnt, respectively;

FIG. 4 is a front view of a burner for burning gaseous fuel to be usedin the method of the invention;

FIG. 5 is a sectional view taken on the line V-V of FIG. 4;

FIG. 6 is a front view of a conventional burner for burning light liquidfuel;

FIG. 7 is a sectional view taken on the line VIIVII of FIG. 6;

FIG. 8 is a front view of a burner for burning light liquid fuel to beused in the method of the invention;

FIG. 9 is a sectional view taken on the line IX-IX of FIG. 8;

FIG. 10 is a front view of a modified burner for burning light liquidfuel to be used in the method of the invention;

FIG. 11 is a sectional view taken on the line XIXI of FIG. 10;

FIG. 12 is a sectional view taken on the line XII-XII of FIG. 11; i

FIG. 13 is a longitudinal sectional view of a burner for burning heavyliquid fuel to be used in the method of the invention;

FIG. 14 is a sectional view taken on the line XIV-XIV of FIG. 13; l

FIG. 15 is a sectional view taken on the line XVXV of FIG. 13;

FIG. 16 is a longitudinal sectional view of a modified burner forburning heavy liquid fuel to be used in the method of the invention; and

FIG. 17 is a cross sectional view of a further modified burner forburning heavy liquid fuel to be used in the method of the invention.

The constructions and operating mechanisms of the burners adapted forpracticing the method of the invention will now be described withreference to these drawings, as follows:

1. The burner suited for the combustionof gaseous fuel in accordancewith this invention is illustrated in FIGS. 4 and 5.

As shown, the burner is made up of a plurality of nozzles 1, each ofwhich consists of a fuel gas feed pipe 2a or 2/1, an air feed pipe 3 forsupplying air with or without the addition of an inert gas (combustiongas), a premixing chamber 4 where air alone or air plus the inert gas ismixed with fuel gas. and a flame-stabilizing baffle 5. i

The fuel gas feed pipes 2a, 2b permit the flow of fuel at ununiform flowrates so that dissimilar excess air rates may be attained.

2. The burner suited for the combustion of light liquid fuel, and itsmodified form, are shown in FIGS. 8 through 12.

The burner is shown as comprising a plurality of nozzles 1', each ofwhich consists of a fuel injection gun 2'a or 2'b, an air feed pipe 3'through which air for combustion or a mixture of air and an inert gas isadmitted, and a flame-stabilizing baffle 4, and is communicated with acommon fuel heater 5'. In addition, where an inert liquid, e.g. water,is to be used; a coldwater spray nozzle 6 is provided.

The fuel injection guns 2a, 2'b are so built as to discharge the fuel atdifferent flow rates.

For the comparison purpose, FIGS. 6 and 7 show a typical combustor of aconventional design for burning light liquid fuel.

3. The burner suited for the combustion of heavy liquid fuel, and itsmodified forms, are shown in FIGS. 13 through I7.

Here, the burner comprises a group of units, i.e., a premixer 21" forauxiliary fuel, a screen burner 2", and heavy oil burners 4 for sprayingheavy oil through the agency of water vapor, a group of units, i.e., aburner cylinder 22" formed with a multiplicity of holes 13" throughwhich the resulting hot gas is cooled by the combustion gas (recycledgas) and is premixed with excessair for combustion, a plurality of airintake plates 3.", and gas passages 16" defined thereby, and a startingoil burner 17".

In the following Examples 1 to 3 will be explained the method oftheinvention wherein heavy oil is sprayed over and mixed with hotcombustion gas arising from the combustion of a gaseous fuel, thusgasifying the heavy oil through contact, and the gas is cooled byrecycled gas to form a combustible gas, and then is burnt in the form oftwo gaseous premixtures of different excess air rates in accordance withthe invention.

EXAMPLE 1 With an apparatus in F I68. 13 to 15, as gaseous fuel (e.g.,LPG) 5 under pressure is premixed with air 6" for combustion, by meansof a premixer 21", and the premixture is completely burnt by a screenburner 2" to form a hot gas 7" at about 2,000C. A number of heavy oilburners 4" for spraying heavy oil 9" with the aid of water vapor 8" aredisposed around the hot gas 7", and by means of the burners 4 the hotgas 7" and the fine droplets of heavy oil 10 (in a mist form) arerapidly mixed up. The mist of fine droplets 10" thus spraying by the oilburners 4" quickly evaporated and undergoes thermal decomposition uponmixing and contact with the hot combustion gas 7" and, through anaqueous gas reaction" with the water vapor present, forms a combustiblegas containing some fine droplets.

The combustible gas thus produced in the gasifying cylinder 1", which isstill as hot as about 1,400C, is cooled by blowing and mixing thereinthe combustion gas (recycled gas) 11 from a furnace outlet not shown,through a multiplicity of holes 13" formed in a burner cylinder 22'. Inthis manner a combustible gas 14 at a relatively low temperature ofabout 500C is obtained.

The recycled gas 11" so added lowers the temperature of the resultinggaseous mixture and thereby serves to preventunintended ignition andfire hazard in the burner in the next stage of the combustiblespremixing with air, while controlling the flame temperature inside theburner at a low level. At the outlet of the burner cylinder 22", inconformity with the present invention, either excess or insufficient air12" is blown into gas passages 16" through a multiplicity of airorifices formed in the burner cylinder 22", so that the gasified fuel14" and air 12" are thoroughly premixed. The gaseous premixture of thegasified fuel, recycled gas, and air, rapidly burns and forms a shortpremix flame 23". v The air intake plates 3" deflect the stream of thegaseous premixture downward and impart swirl to the gas stream, thuseffecting complete premixing of the gasified fuel and air andstabilizing the flame'from the starting oil burner 17" at the time ofstarting. The oil burner 17 has an oil line 18 which is secured to afurnace wall 19".

The experiment indicated that the effect of the flame temperature uponthe production of NOX by the premix flame is greater than when the NOxare produced by a diffused flame.

Combustion in accordance with the present method of a thorough gaseouspremixture of the gasified fuel, air, and recycled gas by a burneraccording to the invention, forms a low-temperature premix flame withremarkably reduced NOx production.

Example 2 EXAMPLE 3 The apparatus employed is the same as used inExample 1 excepting that the orifices 15" open in the gas passages 16"adiffer in number or size of the both from those open in the adjacentpassages 16"b so that the burner can produce gaseous premixtures withlarge and small excess air rates. Accordingly, the excess air rate inthe gas passages 16"a is high, and the rate in the adjacent passages16"b is small.

It has been found by our investigations that the NOx production in apremix flame is small when the excess air rate is extremely low (lessthan 1.0) or extremely high and the production is large with an excessair rate in the medial region. Therefore, the aforedescribedconstruction in which certain gas passages are supplied with a largeexcess of air whereas the remaining passages have a small excess of air,renders it possible to suppress the formation of NOx in the combustiongas and, with an overall excess air rate of greater than 1.0, limit theNOx concentration to a low level.

This is exemplified by an experiment conducted wherein propane gas wasused as the fuel, and 53 per cent of the fuel was mixed with air at anexcess rate of 1.5 to obtain a lean gaseous premixture and the remainingfuel was mixed at an excess air rate of 0.64 to obtain a richpremixture, the overall excess air rate of the pre mixtures being 1.1.An inert gas in the form of recycled gas amounting to 15 percent of thetotal air quantity was divided into two equal portions and added to thetwo premixtures, thus forming two different premixtures both at 270C.These premixtures were issued from the respective nozzles and burnttogether. The NOx content of the exhaust gas could be reduced to 33 ppm(as measured on the dry gas basis).

We claim:

1. A method of burning a fuel by means of a burner which comprisespremixing the fuel with air in an amount larger than the quantitytheoretically required for the combustion of the fuel, allowing theresulting lean premixture to be issued from some of plural nozzles ofthe burner, while at the same time, premixing the fuel with air in anamount less than the theoretically required quantity, allowing theresulting rich premixture to be issued from the remaining nozzles, andburning the two gaseous premixtures together wherein the two gaseouspremixtures are separately mixed with an inert liquid by spraying beforethey are issued from the respective burner nozzles for combustion.

2. A method of burning a fuel by means of a burner which comprisespremixing the fuel with air in an amount larger than the quantitytheoretically required for the combustion of the fuel, allowing theresulting lean premixture to be issued from some of plural nozzles ofthe burner, while at the same time premixing the fuel with air in anamount less than the theoretically required quantity, allowing theresulting rich premixture to be issued from the remaining nozzles, andburning the two gaseous premixtures together wherein the two gaseouspremixtures are separately mixed with a recycled combustion gas beforethey are issued from the respective burner nozzles for combustion.

3. A method of burning a fuel by means of a burner as defined in claim 2wherein the lean premixture is formed to have an excess air rate from1.3 to 1.6 and the rich premixture from 0.5 to 0.8 before they areissued from the respective burner nozzles for combustion.

4. A method of burning a fuel by means of a burner which comprisespremixing the fuel with air in an amount larger than the quantitytheoretically required for the combustion of the fuel, allowing theresulting lean premixture to be issued from some of plural nozzles ofthe burner, while at the same time, premixing the fuel with air in anamount less than the theoretically required quantity, allowing theresulting rich premixture to be issued from the remaining nozzles, andburning the two gaseous premixtures together wherein the fuel is agasified fuel prepared by thermally decomposing a heavy liquid fuel,adding an inert gas or liquid to the decomposed fuel, and cooling themixture.

5. A burner comprising gaseous or gasified fuel and air conduits, firstpremixing chambers for forming a fuel lean premixture consisting of thefuel and air in an amount larger than the quantity theoreticallyrequired for the combustion of the fuel, second premixing chambers forforming a fuel-rich premixture consisting of the fuel and air in anamount smaller than the theoretically required quantity, first nozzlesfor injecting the lean premixture, second nozzles for injecting the richpremixture, and conduits for an inert fluid means for discharging aninert fluid for mixing with the air to be used for the combustion.

6. A burner as defined in claim 5 further comprising heating means forgasifying a light liquid fuel.

7. A burner as defined in claim 5, further comprising a gasifyingcylinder for converting heavy liquid fuel to a mixture of combustiblegases, a premixer for auxiliary fuel, a screen burner for burning theauxiliary fuel, burners for spraying the heavy liquid fuel through theaid of steam, a burner cylinder fomled with holes for supplying air tobe mixed with a hot gas generated in the gasifying cylinder, means forcooling the hot gas formed in the gasifying cylinder by supplying andmixing therewith an inert fluid, air intake plates, gas passages definedby the air intake plates, and a starting oil burner.

8. A burner as defined in claim 7 wherein said means for cooling theresulting hot gas consists of a plurality of holes formed on the burnercylinder and conduit connected thereto through which the combustion gasis recycled.

9. A burner as defined in claim 7 wherein said means for cooling theresulting hot gas consists of a feed water pipe provided with waterspraying nozzles inside the gasifying cylinder.

1. A method of burning a fuel by means of a burner which comprisespremixing the fuel with air in an amount larger than the quantitytheoretically required for the combustion of the fuel, allowing theresulting lean premixture to be issued from some of plural nozzles ofthe burner, while at the same time, premixing the fuel with air in anamount less than the theoretically required quantity, allowing theresulting rich premixture to be issued from the remaining nozzles, andburning the two gaseous premixtures together wherein the two gaseouspremixtures are separately mixed with an inert liquid by spraying beforethey are issued from the respective burner nozzles for combustion.
 2. Amethod of burning a fuel by means of a burner which comprises premixingthe fuel with air in an amount larger than the quantity theoreticallyrequired for the combustion of the fuel, allowing the resulting leanpremixture to be issued from some of plural nozzles of the burner, whileat the same time premixing the fuel with air in an amount less than thetheoretically required quantity, allowing the resulting rich premixtureto be issued from the remaining nozzles, and burning the two gaseouspremixtures together wherein the two gaseous premixtures are separatelymixed with a recycled combustion gas before they are issued from therespective burner nozzles for combustion.
 3. A method of burning a fuelby means of a burner as defined in claim 2 wherein the lean premixtureis formed to have an excess air rate from 1.3 to 1.6 and the richpremixture from 0.5 to 0.8 before they are issued from the respectiveburner nozzles for combustion.
 4. A method of burning a fuel by means ofa burner which comprises premixing the fuel with air in an amount largerthan the quantity theoretically required for the combustion of the fuel,allowing the resulting lean premixture to be issued from some of pluralnozzles of the burner, while at the same time, premixing the fuel withair in an amount less than the theoretically required quantity, allowingthe resulting rich premixture to be issued from the remaining nozzles,and burning the two gaseous premixtures together wherein the fuel is agasified fuel prepared by thermally decomposing a heavy liquid fuel,adding an inert gas or liquid to the decomposed fuel, and cooling themixture.
 5. A burner comprising gaseous or gasified fuel and airconduits, first premixing chambers for forming a fuel lean premixtureconsisting of the fuel and air in an amount larger than the quantitytheoretically required for the combustion of the fuel, second premixingchambers for forming a fuel-rich premixture consisting of the fuel andair in an amount smaller than the theoretically required quantity, firstnozzles for injecting the lean premixture, second nozzles for injectingthe rich premixture, and conduits for an inert fluid means fordischarging an inert fluid for mixing with the air to be used for thecombustion.
 6. A burner as defined in claim 5 further comprising heatingmeans for gasifying a light liquid fuel.
 7. A burner as defined in claim5, further comprising a gasifying cylinder for converting heavy liquidfuel to a mixture of combustible gases, a premixer for auxiliary fuel, ascreen burner for burning the auxiliary fuel, burners for spraying theheavy liquid fuel through the aid of steam, a burner cylinder formedwith holes for supplying air to be mixed with a hot gas generated in thegasifying cylinder, means for cooling the hot gas formed in thegasifying cylinder by supplying and mixing therewith an inert fluid, airintake plates, gas passages defined by the air intake plates, and astarting oil burner.
 8. A burner as defined in claim 7 wherein saidmeans for cooling the resulting hot gas consists of a plurality of holesformed on the burner cylinder and conduit connected thereto throughwhich the combustion gas is recycled.
 9. A burner as defined in claim 7wherein said means for cooling the resulting hot gas consists of a feedwater pipe provided with water spraying nozzles inside the gasifyingcylinder.